The present invention relates to modulation and encoding/decoding of signals. More specifically, the invention relates to a novel method and apparatus for obtaining flexible-rate/bandwidth-efficient encoding and decoding plus modulation.
Several techniques for combining concatenated convolution codes with interleaver and spectrally efficient modulations have been proposed, due to the attractiveness of achieving high spectral efficiency and high coding gain at the same time.
Concatenated convolutional codes with interleaver (known as xe2x80x9cturbo codesxe2x80x9d in their parallel concatenated structure) represent one of the most important results in code theory achieved during the last decade; in fact, the performance of optimized schemes can be within 1 dB from the theoretical Shannon limit. The main ingredients of concatenated codes with interleaver consist of two Convolutional Codes (CCs) and one interleaver. The impressive performance of those codes is achieved through an iterative decoding process that refines the decisions taken by each constituent decoding block. Among the turbo codes two different classes can be distinguished depending on the concatenation scheme.
The first class is based on Parallel Concatenation: the information sequence enters the first systematic encoder, which generates a coded sequence; at the same time, the information sequence is transformed by the interleaver into a permutation of itself and is successively encoded by the second systematic encoder. A block diagram of the encoder in question is shown in FIG. 1. The code sequence of the parallel turbo code is obtained through concatenation, in some suitable order, of the two code sequences. This turbo code was first proposed in 1993 in C. Berrou, A. Glavieux and P. Thitimajshima, xe2x80x9cNear Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes,xe2x80x9d in Proceedings of ICC ""93 (Geneva, Switzerland), pp. 1064-1070, May 1993, and further described in U.S. Pat. No. 5,446,747.
The second class is based on Serial Concatenation, as first proposed in S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, xe2x80x9cSerial concatenation of interleaved codes: performance analysis, design and iterative decoding,xe2x80x9d IEEE Transactions on Information Theory, pp. 909-926, May 1998, which is incorporated herein by reference. In this case the output of the outer encoder is encoded by the inner encoder, after a permutation through a suitable interleaver. A block diagram of such an encoder is shown in FIG. 2.
The constituent codes (C1, C2) can be block or convolutional codes for both classes (parallel and serial).
Pragmatic Trellis-Coded Modulation (PTCM), proposed in the work by Andrew Viterbi, et al., xe2x80x9cA Pragmatic Approach to Trellis-Coded Modulation,xe2x80x9d IEEE Communications Magazine, July 1989, pp. 11-19, is a combined error-correction coding and modulation scheme that utilizes a standard convolutional code applied to certain bits of M-ary Phase Shift Keyed (M-PSK) or M-ary Quadrature Amplitude Modulation (M-QAM) symbol mappings.
The earlier studies on concatenated codes with interleavers dealt with parallel concatenated convolutional codes (PCCC) with medium-low rates (xc2xd, ⅓ and below) used in conjunction with binary (or, equivalently, independent quaternary) modulations, such as 2-PAM, BPSK, QPSK, with the aim of obtaining very large coding gains. Subsequently, several attempts have been undertaken to merge PCCC with high-level modulation schemes in order to conjugate significant coding gains with high spectral efficiencies. The first paper on this subject is S. Le Goff, A. Glavieux and C. Berrou, xe2x80x9cTurbo-codes and high spectral efficiency modulation,xe2x80x9d in Communications, 1994. ICC ""94, SUPERCOMM/ICC ""94, Conference Record, xe2x80x9cServing Humanity Through Communications.xe2x80x9d IEEE International Conference on, pp. 656-649 vol.2, 1994, which adopts a pragmatic approach to obtain 2,3 and 4 bits/signal by means of 8PSK and 16QAM modulations with turbo codes (PCCC schemes).
Several techniques to merge turbo codes with classical Ungerboeck Trellis-Coded Modulation (TCM) are described in the literature for both parallel and serial concatenated convolutional coding (SCCC) schemes.
Exemplary of the former coding schemes are the works by P. Robertson and T. Worz, xe2x80x9cBandwidth-efficient turbo trellis-coded modulation using punctured component codes,xe2x80x9d IEEE Journal on Selected Areas in Communications, vol. 16 2, pp. 206-218, February 1998, and S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, xe2x80x9cParallel concatenated trellis coded modulation,xe2x80x9d in Proceedings of ICC ""96, (Dallas, Tex.), June 1996.
Exemplary of the latter coding schemes are the works S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, xe2x80x9cSerial concatenated trellis coded modulation with iterative decoding: design and performance,xe2x80x9d in Proceedings of GLOBECOM ""97xe2x80x94Communications Theory Miniconference, (Phoenix, Ariz.)., October 1997, and S. Benedetto, D. Divsalar, R. Garello, G. Montorsi and F. Pollara, xe2x80x9cSelf-Concatenated Trellis Coded Modulation with Self-Iterative Decoding,xe2x80x9d in GLOBECOM ""98xe2x80x94Communications Theory Miniconference, (Sydney, Australia), October 1998.
Also, references of interest are U.S. Pat. No. 6,088,387 to Gelblum et al., and U.S. Pat. No. 6,023,783 to Divsalar et al.
Specifically, this latter patent discloses several improved turbo code apparatuses and methods encompassing several classes such as:
a data source is applied to two or more encoders with an interleaver between the source and each of the second and subsequent encoders. Each encoder outputs a code element which may be transmitted or stored. A parallel decoder provides the ability to decode the code elements to derive the original source information without the use of a received data signal corresponding to such information. The output may be coupled to a multilevel trellis-coded modulator (TCM);
a data source is applied to two or more encoders with an interleaver between the source and each of the second and subsequent encoders. Each of the encoders outputs a code element. In addition, the original data source is output from the decoder. All of the output elements are coupled to a TCM;
at least two data sources are applied to two or more encoders with an interleaver between each source and each of the second and subsequent encoders. The output may be coupled to a TCM;
at least two data sources are applied to two or more encoders with at least two interleavers between each source and each of the second and subsequent encoders; and
at least one data source is applied to one or more serially linked encoders through at least one interleaver; the output may be coupled to a TCM.
Specifically, the solution described in the captioned patent includes a novel way of terminating a turbo coder.
SCCCs with interleavers have been shown to yield some advantages with respect to PCCCs, especially when very low bit error probabilities are being pursued: see in that respect the first work by Benedetto et al. referred to in the foregoing.
Besides, the need of combining large coding gains with high spectral efficiency is becoming increasingly important, owing to applications for which both power and bandwidth are precious and scarce resources. Moreover, applications in which the channel reliability is subject to large variations ask for coding-modulation schemes able to move back and forth the trade-off point between bandwidth and power efficiency in favor of either resource. In other words, rather than precisely tuned designs, those applications require versatile schemes yielding good, though not optimized, performance. For example, this is the case of wireless communications, like third generation cellular communications and digital video broadcasting.
Thus, the prior art fails to propose a xe2x80x9cuniversalxe2x80x9d encoding/decoding scheme that can yield the very good bit error rate performances of the SCCCs for a wide range of spectral efficiencies, and with the same hardware implementation.
An embodiment the present invention is thus to combine SCCC and two-dimensional modulation schemes (like M-PSK and M-QAM) to obtain a versatile structure for both encoder and decoder that yields the aforementioned advantages with respect to the solutions so far presented in the literature.
The embodiment provides a flexible-rate scheme yielding theoretical performance, in terms of coding gain, very close to the best bandwidth-efficient xe2x80x9cad hocxe2x80x9d optimized schemes, based on SCCC and classical TCM, presented in literature so far.
The embodiment further provides a set of rules for choosing the constituent convolutional codes, the structure of their interleaver, their puncturing rates and algorithms, the mapping to the two-dimensional modulator and the decoder architecture as well, so that a single encoder/decoder pair can be utilized on the basis of the desired spectral efficiency range.
The embodiment employs a serially concatenated convolutional code (SCCC) in conjunction with two-dimensional modulation schemes in a way different from the classical TCM schemes.
An innovative feature of the embodiment is its flexibility and versatility, as a high and versatile spectral efficiency can be achieved acting on a small set of parameters and employing the same architectural framework.
Irrespective of this, the performance does not present a significant degradation with respect to schemes optimized xe2x80x9cad hocxe2x80x9d for a given spectral efficiency. Moreover, the SCCC proposed implementation contains itself original ideas regarding some parts of the architecture, aimed at ensuring the above-mentioned flexibility of the scheme.
For the same reason, rather than a specific implementation of a digital transmission scheme including an error correcting code, a modulator and the receiving counterpart, the present embodiment primarily relates to original apparatus, and related set of design rules for its basic blocks, allowing to achieve the aforementioned advantages and goals.